protected function garbage_collect()
 {
     $lifetime = Kohana::config('stash.lifetime');
     $lifetime = $lifetime > 0 ? $lifetime : 7200;
     $oldest = time() - $lifetime;
     foreach (glob(stash::get_directory() . '*') as $file) {
         if (filectime($file) < $oldest) {
             unlink($file);
         }
     }
 }